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12. Kelp, Urchins, and Otters in the California Coastal Region
The near-extirpation and subsequent rebound of the sea otter has
amounted to a large-scale biological experiment. This is becau se the sea
otter is both a keystone species and a top-level pred ator in the kelp forest
community. One of the preferred food sources of the sea otter is sea
urchins (Strongylocentrotus spp.), which also happen to be the major herbivore of the kelp forest. With the removal of sea otter, several kelp forests
on the Californian and Alaskan coastlines have succumbed to intense grazing pressures by sea urchins .
Under normal conditions, kelp fronds have a lifetime of six months , after
which they die and become detached from the parent plant (Monterey Bay
Aquarium Research Institute 2000). Urchins tend to consume this drift kelp
because it allows them to remain sedentary and, therefore, not much energy is required for foraging activities. As the urchin population rises, however, the stock of drift kelp may become inadequate and urchins begin to
actively graze on the kelp plant itself, often cavitating the holdfasts and destroying the entire plant (Wantanabe and Harrold 1991). Urchins have been
known to move collectively in "fronts" quickly forming kelp-free areas
known as "barrens." These barren areas can be very extensive because
urchin grazing can prevent kelp re-establishment. The removal of sea otters
has led to the formation of several of these barrens along the Californian
coast.
Once exploitation pressures were lifted for sea otters, they began to migrate back into their historical ranges . There , in the emptiness of the barrens, urchins are easy prey. Otters have a voracious appetite, consuming
20-30% of their body weight per day in order to maintain their high metabolism, which they requ ire due to the lack of a blubber layer (Riedman and
Estes 1988). According to one study, urchins were "virtually absent" two
years after recolonization with otters (Estes and Duggins 1985). This absenc e of urchin s has enabled kelp to become re-established in many of the
areas that were once barrens. This has helped to restore the original dynamic equilibrium of the kelp forests.
The dynamics of the kelp-urchin-otter equilibrium has interested scientists in recent years. The recolonization of otters has given scientists the
opportunity to witness two biological phenomena, the trophic interactions
in the kelp-urchin-otter food web, and how the ecosystem responds to
what is, essentially, an invading species. By examining the effect that otter
reintroduction has on the local ecosystem, scientists hope to infer what
might happen in similar ecosystems that are being invaded by foreign
species.
The reintroduction of otters also has impacts on local economies, as the
presence of sea otters affects both urchin and kelp industries as well as fisheries, whose shellfish and finfish rely on kelp for protecti on against predators. The purpose of the model developed in this chapter is to explore interactions among kelp , urchins and otters and to quantify changes in
biomass of urchin and kelp populations following otter reintroduction.
12. Kelp, Urchins, and Otters in the California Coastal Region
The near-extirpation and subsequent rebound of the sea otter has
amounted to a large-scale biological experiment. This is becau se the sea
otter is both a keystone species and a top-level pred ator in the kelp forest
community. One of the preferred food sources of the sea otter is sea
urchins (Strongylocentrotus spp.), which also happen to be the major herbivore of the kelp forest. With the removal of sea otter, several kelp forests
on the Californian and Alaskan coastlines have succumbed to intense grazing pressures by sea urchins .
Under normal conditions, kelp fronds have a lifetime of six months , after
which they die and become detached from the parent plant (Monterey Bay
Aquarium Research Institute 2000). Urchins tend to consume this drift kelp
because it allows them to remain sedentary and, therefore, not much energy is required for foraging activities. As the urchin population rises, however, the stock of drift kelp may become inadequate and urchins begin to
actively graze on the kelp plant itself, often cavitating the holdfasts and destroying the entire plant (Wantanabe and Harrold 1991). Urchins have been
known to move collectively in "fronts" quickly forming kelp-free areas
known as "barrens." These barren areas can be very extensive because
urchin grazing can prevent kelp re-establishment. The removal of sea otters
has led to the formation of several of these barrens along the Californian
coast.
Once exploitation pressures were lifted for sea otters, they began to migrate back into their historical ranges . There , in the emptiness of the barrens, urchins are easy prey. Otters have a voracious appetite, consuming
20-30% of their body weight per day in order to maintain their high metabolism, which they requ ire due to the lack of a blubber layer (Riedman and
Estes 1988). According to one study, urchins were "virtually absent" two
years after recolonization with otters (Estes and Duggins 1985). This absenc e of urchin s has enabled kelp to become re-established in many of the
areas that were once barrens. This has helped to restore the original dynamic equilibrium of the kelp forests.
The dynamics of the kelp-urchin-otter equilibrium has interested scientists in recent years. The recolonization of otters has given scientists the
opportunity to witness two biological phenomena, the trophic interactions
in the kelp-urchin-otter food web, and how the ecosystem responds to
what is, essentially, an invading species. By examining the effect that otter
reintroduction has on the local ecosystem, scientists hope to infer what
might happen in similar ecosystems that are being invaded by foreign
species.
The reintroduction of otters also has impacts on local economies, as the
presence of sea otters affects both urchin and kelp industries as well as fisheries, whose shellfish and finfish rely on kelp for protecti on against predators. The purpose of the model developed in this chapter is to explore interactions among kelp , urchins and otters and to quantify changes in
biomass of urchin and kelp populations following otter reintroduction.